The present invention relates to the use of polyacrylic acid-derived polymers and inorganic metal salts as a grinding aid system to increase the rate of coal or ore-particle breakage in the wet-grinding of solids in mills such as ball, bead, rod or pebble mills, or in autogenous grinding operations.
In the processing of mineral ores and many other solids, the essential step is the comminution of the solids down to the size at which valuable mineral grains are released from the gangue matrix. With the inevitable trend towards working of lower-grade ore deposits, the amounts of minerals liberated tend to decrease and the grinding cost per ton of product increases. This factor alone constitutes a considerable fraction of the overall cost of winning metals and the increase in cost of energy has made grinding costs a very significant factor.
The amount of breakage per unit of time (breakage kinetics) and mass transfer of grinding mineral ores are usually controlled by the addition and removal of water, an excellent medium because of its high polarity, to the mill. When the mass transport of the slurry through the mill decreases, the mill operator takes corrective action by either decreasing the solids feed rate and/or temporarily increasing the amount of water entering the mill. While both actions will prevent the mill from overloading, mill efficiency is reduced because fewer solids are ground per unit of time under such conditions.
Additionally, it is well known that the traditional tumbling mill apparatus used for wet-grinding ores are extremely inefficient in energy utilization, wasting (based on theoretical bond breakage energies) perhaps as much as 90% or more of the energy supplied to the mill. Therefore, even small increases of a few percent in the reduction of size distribution of ore particles and an increase in throughput of ore ground per unit of time would significantly improve the efficiency of grinding and cost of mill operations, especially with respect of energy utilization.
While various methods and chemical agents that act as grinding aids have been employed in efforts to increase grinding efficiencies and economics, these efforts have at best been only partially beneficial and many have even proved to be contradictory in related downstream processing operations. Various chemical agents, e.g., dispersants, surfactants, polysiloxane, organosilicones, glycols, amines, graphite, non-polar liquids and the like have all been utilized and may increase the rate of grinding by preventing particle agglomeration. However, as reported in Perry's Chemical Engineering Handbook, 5th Ed. 1973, at Sec. 8-12, there really is no scientific method of choosing the most effective surfactant. Rather, surfactant lists and kits that can be used for systematic trails are made available.
Chemical agents, such as polyacrylic acid salts, copolymers of acrylic acid and acrylamide, hydrolyzed polyacrylonitrile and the like are known to be useful as dispersants at low molecular weight ranges. Polymers having a molecular weight from a few thousand up to about 50,000, for example, have been utilized as dispersants in the grinding of calcium carbonate to separate impurities therefrom. See U.S. Pat. Nos. 3,534,911 and 3,604,635.